1. Field of the Invention
The present invention relates to integrated circuit transistors. In particular, the invention relates to a structure and method of fabricating a transistor having a trench gate.
2. Description of the Related Art
Integrated circuit designers often desire to increase the density of elements within an integrated circuit by reducing the size of the individual elements and reducing the separation distance between neighboring elements. One example of a common integrated circuit element is a transistor, which can be found in many devices, such as memory circuits, processors, and the like. A typical integrated circuit transistor comprises a source, a drain, and a gate formed at the surface of the substrate.
Although it is generally desirable to reduce the size of integrated circuit transistors, the ability to shrink certain dimensions, such as the length of the gate, can be limited due to the voltage levels needed to perform certain operations. In one example, a relatively high amount of voltage can be required by some transistors used in flash memory to perform certain operations, such as program, read, and erase. One approach for reducing the size of such transistors while maintaining the gate length necessary to satisfy the voltage requirements is to form the transistor gate as a trench below the surface of the substrate.
In one example, a transistor gate can be implemented as a U-shaped trench connecting the source and the drain. Such a U-shaped gate trench maintains the gate length while allowing the gate surface area to decrease.